Regulation of delta-opioid receptor mRNA levels by receptor-mediated and direct activation of the adenylyl cyclase-protein kinase A pathway.

The effects of activation of the adenylyl cyclase-protein kinase A pathway on the expression of delta-opioid receptor mRNA in the NG108-15 neuroblastoma x glioma cell line has been investigated. Activation of prostaglandin E1 (PGE1) receptors, which are positively coupled to adenylyl cyclase, resulted in a reduction in delta-receptor messenger RNA levels. Direct stimulation of adenylyl cyclase by forskolin or treatment of cells with the cyclic AMP analogue dibutyryl cyclic AMP (db-cAMP) mimicked the effect of PGE1. Down-regulation in receptor protein levels, as measured by loss of radioligand binding sites, was also observed and its extent correlated well with the decrease in the amount of delta-opioid receptor transcripts. D-Ser2-Leu-enkephalin-Thr6 (DSLET) inhibition of adenylyl cyclase activity was also diminished after db-cAMP treatment. Inhibitors of protein kinase A (PKA) partially reversed the PGE1- and db-cAMP-mediated repression of the delta-opioid receptor mRNA levels. The rate of degradation of delta-opioid receptor mRNA in the presence of actinomycin D was not altered in response to db-cAMP, suggesting that mRNA stability is not reduced by PKA action. The regulation of delta-opioid receptor mRNA levels by db-cAMP was not sensitive to the protein synthesis inhibitor cycloheximide, suggesting that de novo protein synthesis is not required in this process.